Police are trying to learn why an SUV they believe was carrying a family of eight plunged off a cliff in Northern California. Jennifer and Sarah Hart from Washington state and three of their kids were found dead at the scene but the other three children are still missing. Police found the family's vehicle Monday after a passerby spotted it at the bottom of the cliff. Authorities don't know what caused the crash but they believe all six kids, who were adopted, were inside the car and none were wearing seat belts, reports CBS News' Anne-Marie Green.

Mendocino County Sheriff Tom Allman said there were no skid or brake marks where the Hart family's car plunged 100 feet off a cliff. According to police, the SUV pulled off on a dirt turnout and then traveled about 75 feet before going over the cliff's edge, and into the Pacific Ocean.

Some family friends describe the Harts as tight-knit, but others noticed problems. Bruce and Dana Dekalb, who lived next door, said they worried about the kids.

This March 20, 2016 photo shows Hart family of Woodland, Wash., at a Bernie Sanders rally in Vancouver, Wash. Authorities in Northern California say they believe all six children were in a vehicle that plunged off a coastal cliff.
Tristan Fortsch/KATU News via AP

"They portrayed this happy little family and yet their daughter is telling us please, please, please, begging us not to go back they're abusing her and then Devonte coming over here and telling us that he's being starved to death," Dana, who called Child Protective Services on Friday, said. "I was trying to help them and protect them and this is the result."

Police say they are still searching for 16-year-old Hannah, 12-year-old Sierra, and 15-year-old Devonte Hart. Devonte gained national attention in 2014 when he was photographed with tears in his eyes hugging a Portland police officer at a protest.

In 2011, Sarah Hart pleaded guilty in Minnesota to domestic assault. According to court records, a teacher saw bruises on the couple's 6-year-old daughter. The girl told the teacher, "mom hit me." Sarah Hart allegedly admitted to police she "let her anger get out of control."

Neighbors say they saw the family rush out of their home Friday after Child Protective Services knocked on their door. The sheriff said there's no reason to believe this was intentional and they'll continue to investigate. He is asking anyone who knew the family, or their last movements, to come forward.